Itar-Tass news agency quoted Lakhonin as saying: "Interpol will be called on, and is already being called on, for the speedy exchange of information between various countries."
A spokeswoman for the French-based Interpol said the case was international and "Interpol can therefore offer and provide international assistance between the countries, as and when required".
The three investigations have sent British investigators to Russia to question people who met Litvinenko, and Russian newspapers have reported that Russian prosecutors might be preparing to fly to London to conduct interviews.
A German prosecutor has said Russian businessman Dmitry Kovtun, who met Litvinenko on the day he fell ill and who is now in hospital in Moscow, could be a possible suspect in the case. Kovtun denies any part in Litvinenko's poisoning.
Gerald Kirchner of the German Radiation Protection Agency said four people feared to have been contaminated with polonium 210 through contact with Kovtun were unlikely to suffer health problems.
"This is not surprising, just look in history and you can find many cases such as Litvinenko's."
Z. F. Khan, Paris, France
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The four were Kovtun's ex-wife, her current partner and their two children. Kovtun spent the night of October 28 at her Hamburg apartment, where traces of the radioactive substance were found.
Kirchner told German radio: "The contamination in the apartment is roughly equal to what a smoker would be exposed to if he smoked several packs of cigarettes or strong cigars."
Hamburg police said they were sending urine samples from the four to a special laboratory in Dresden to determine whether they had polonium 210 contamination inside their bodies.
NDR radio reported that they said they had requested help from their Moscow counterparts in connection to information about Kovtun's stay in Germany between October 28 and November 1. At the weekend they said earlier requests had received no response.
Kovtun has been in a Moscow hospital with symptoms of radiation poisoning but on Tuesday he told Russia's First Channel he was feeling better and would be leaving soon.
"Doctors say the situation is stable now and I'm getting better," he told the television station by telephone. "I think I will stay in hospital another week or 10 days."
Litvinenko died in a London hospital on November 23 after exposure to radioactive polonium 210. In a statement associates released after his death, he accused Vladimir Putin, the Russian president, of ordering his killing. The Kremlin denies involvement.